1. Field of the Invention
This application relates to devices and methods for harvesting of marine bivalve mollusks. More particularly it concerns devices and methods for the harvesting of clams with minimum damage to the environment.
2. Description of the Prior Art
Submarine farming is an extremely complicated undertaking because the marine environment is many times more complex than traditional terra firma agriculture farming. Hence, three major labor intensive phases of such an endeavor, planting (or stocking), nurturing and harvesting must be accomplished with even greater attention paid to the consequences of methodology and equipment design.
The submarine interface between bottom sediment and the water just above it is a very important region of any body of water. Physically this is a result of currents, sediment transport, etc. in the area over a period of time. Grain size, stratification and contour reflect an established balance. Man's alterations result in processes which will seek to re-establish this balance. A change in contour, for example, will mean increased sediment transport in the area, increasing turbidity levels, changes in light transmission characteristics and biological productivity.
It has been established that a delicate chemical balance exists, at least with organic nutrients, between the available nutrients in the water and nutrients trapped in the sediment. This "bank" is important to the long term biological productivity of the area. It is believed that some toxic materials, heavy metals, and other containments are trapped in the sediment, removing them as a hazard to most of the food chain.
Biologically, the bottom interface is the home of a diverse community, containing elements of the entire food chain, from bacteria to predator. The particular area of concern here is the infauna, or animal population living in the sediments just below the interface. Some members of the population are of commercial value, a good example being the hard clam, Mercenaria mercenaria.
The clam is a filter-feeding shellfish. This animal, encased in a pair of hard, protective shells, positions itself just below the interface and extends its siphon or "neck" into the water above. Water is pumped through the clam and expelled through a second passage in the siphon. Food, mostly microalgae, is strained from the water and utilized. At the same time, oxygen necessary for respiration is obtained from the pumped water. The clam has no parallel in agriculture. It would constitute a hard-shelled animal, buried like a potato, deriving both its nutritional and respiration needs from breathing air.
While the clam has many natural predators, man is perhaps the most serious. Harvesting of clams in the wild ranges from digging by hand to the use of powerful ships and specialized mechanical equipment. Traditional manual harvest tools include rakes, tongs and the like. Harvesting clams with such tools becomes commercially inefficient because of their inefficiency under conditions of high clam densities. Also with use of such tools, it is difficult to assure that all of the planted area has been fully harvested.
Mechanical harvesting equipment currently available include hydraulic dredges (see U.S. Pat. No. 3,462,858) and suction dredges (see U.S. Pat. No. 3,624,932). However, with such equipment, the substrate is removed and then the shellfish are recovered. This results in undesirable habitat damage.
Another type of shellfish harvesting equipment less destructive to the habitat are sledge-type devices that comprise inclined tines or teeth which penetrate the substrate (see U.S. Pat. Nos. 4,112,602; 4,425,723 & 4,827,635). The present invention relates to this class of shellfish harvesters and provides improvements in the construction and operation thereof.
For example, the amount of energy required to drag sledge-type devices of the prior art through dense clam beds is formidable thereby limiting the size of the rake that can be incorporated in the sledge. Also, any propeller driven craft requires considerable thrust to pull such sledge devices forward. In shallow water, this can resuspend bottom sediments and increase turbidity. The present invention makes it possible to reduce the drag created by a given size rake contained in sledge-type shellfish harvesting devices.
All of the prior methods and equipment, including digging by hand, accomplish their goal by disturbing, and in most cases removing, much more sediment than surrounds the clam itself.
If one could grow a clam with a string attached, and harvest was accomplished by pulling on that string, the bottom interface would be disturbed, but the disturbance would be limited to that which was absolutely necessary to remove the clam. While this is not practical, the harvest of infauna should be pursued in that light in view of the extreme importance and delicacy of the region in which they exist.
The culture of shellfish as a "crop" compounds the problem, and again, the hard clam will serve as a good example. The density of clams in the wild may be several per square foot. As in agriculture, the farmer needs maximum yield to succeed in business, so juvenile clams ("seed") are planted in densities up to 100 per square foot, the "beds" are covered with nets to discourage natural predators while allowing water circulation. These nets are cleaned and changed as necessary, much as the dry land farmer protects his crop from weeds, birds, and other factors which constitute competition for his crop. While the farmer (or rancher) of the land is equipped for efficient harvest, the aquatic farmer harvesting infaunal organisms is not. The equipment available does not reflect the complexities of this new environment. The equivalent may be likened to a potato farmer harvesting his fields with a bulldozer, then separating the potatoes from the pile of dirt.
Harvest size clams, planted in the densities described, form a "bed" similar to a cobblestone road bed. Harvesting by hand is difficult due to the density. The "ideal density" which will produce a sustainable harvest over a long period of time has not been determined. Meanwhile, an environmentally benign method of harvesting and novel equipment for carrying out the method has been the focus of this present invention.